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How do good metals go bad?

New measurements have solved a mystery in solid state physics: How is it that certain metals do not seem to

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New measurements have solved a mystery in solid state physics: How is it that certain metals do not seem to adhere to the valid rules?

We all have a clear picture in mind when we think of metals: We think of solid, unbreakable objects that conduct electricity and exhibit a typical metallic sheen. The behaviour of classical metals, for example their electrical conductivity, can be explained with well-known, well-tested physical theories.

But there are also more exotic metallic compounds that pose riddles: Some alloys are hard and brittle, special metal oxides can be transparent. There are even materials right at the border between metal and insulator: tiny changes in chemical composition turn the metal into an insulator – or vice versa. In such materials, metallic states with extremely poor electrical conductivity occur; these are referred to as “bad metals”. Until now, it seemed that these “bad metals” simply could not be explained with conventional theories. New measurements now show that these metals are not that “bad” after all. Upon closer inspection, their behaviour fits in perfectly with what we already knew about metals.

Small change, big difference

Prof. Andrej Pustogow and his research group at the Institute for Solid State Physics at TU Wien (Vienna) are conducting research on special metallic materials – small crystals that have been specially grown in the laboratory. “These crystals can take on the properties of a metal, but if you vary the composition just a little bit, we are suddenly dealing with an insulator that no longer conducts electricity and is transparent like glass at certain frequencies,” says Pustogow.

Right at this transition, one encounters an unusual phenomenon: the electrical resistance of the metal becomes extremely large – larger, in fact, than should be possible at all according to conventional theories. “Electrical resistance has to do with the electrons being scattered at each other or at the atoms of the material”, explains Andrej Pustogow. According to this view, the greatest possible electrical resistance should occur if the electron is scattered at every single atom on its way through the material – after all, there is nothing between an atom and its neighbour that could throw the electron off its path. But this rule does not seem to apply to so-called “bad metals”: They show a much higher resistance than this model would allow.

It all depends on the frequency

The key to solving this puzzle is that the material properties are frequency-dependent. “If you just measure the electrical resistance by applying a DC voltage, you only get a single number – the resistance at zero frequency,” says Andrej Pustogow. “We, on the other hand, made optical measurements using light waves with different frequencies.”

This showed that the “bad metals” are not so “bad” after all: At low frequencies they hardly conduct any current, but at higher frequencies they behave as one would expect from metals. The research team considers tiny amounts of impurities or defects in the material, that can no longer be adequately shielded by a metal at the boundary to an insulator, as a possible cause. These defects can cause some areas of the crystal to no longer conduct electricity because there the electrons remain localized in a certain place instead of moving through the material. If a DC voltage is applied to the material so that the electrons can move from one side of the crystal to the other, then virtually every electron will eventually hit such an insulating region and current can hardly flow.

At high AC frequency, on the other hand, every electron moves back and forth continuously – it does not cover a long distance in the crystal because it keeps changing direction. This means that in this case many electrons do not even come into contact with one of the insulating regions in the crystal.

Hope for important further steps

“Our results show that optical spectroscopy is a very important tool for answering fundamental questions in solid-state physics,” says Andrej Pustogow. “Many observations for which it was previously believed that exotic, novel models had to be developed could very well be explained by existing theories if they were adequately extended. Our measurement method shows where the additions are necessary.” Already in earlier studies, Prof. Pustogow and his international colleagues gained important insight into the boundary region between metal and insulator using spectroscopic methods, thus establishing a fundament for theory.

The metallic behaviour of materials subject to strong correlations between the electrons is also particularly relevant for so-called “unconventional superconductivity” – a phenomenon that was discovered half a century ago but is still not fully understood.

###

Contact

Ass. Prof. Dr. Andrej Pustogow

Institute for Solid State Physics

TU Wien

+43 1 58801 13128

[email protected]

http://www.ifp.tuwien.ac.at/forschung/pustogow-research/home

https://www.tuwien.at/en/tu-wien/news/news-articles/news/wie-werden-gute-metalle-schlecht

Right at this transition, one encounters an unusual phenomenon: the electrical resistance of the metal becomes extremely large – larger, in fact, than should be possible at all according to conventional theories. “Electrical resistance has to do with the electrons being scattered at each other or at the atoms of the material”, explains Andrej Pustogow. According to this view, the greatest possible electrical resistance should occur if the electron is scattered at every single atom on its way through the material – after all, there is nothing between an atom and its neighbour that could throw the electron off its path. But this rule does not seem to apply to so-called “bad metals”: They show a much higher resistance than this model would allow.

Source: https://bioengineer.org/how-do-good-metals-go-bad/

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Bioengineer

The science of sound, vibration to better diagnose, treat brain diseases

Multidisciplinary researchers uncover new ways to use ultrasound energy to image and treat hard-to-reach areas of brainCredit: Allison Carter, Georgia

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Multidisciplinary researchers uncover new ways to use ultrasound energy to image and treat hard-to-reach areas of brain

A team of engineering researchers at the Georgia Institute of Technology hopes to uncover new ways to diagnose and treat brain ailments, from tumors and stroke to Parkinson’s disease, leveraging vibrations and ultrasound waves.

The five-year, $2 million National Science Foundation (NSF) project initiated in 2019 already has resulted in several published journal articles that offer promising new methods to focus ultrasound waves through the skull, which could lead to broader use of ultrasound imaging — considered safer and less expensive than magnetic resonance imaging (MRI) technology.

Specifically, the team is researching a broad range of frequencies, spanning low frequency vibrations (audio frequency range) and moderate frequency guided waves (100 kHz to 1 MHz) to high frequencies employed in brain imaging and therapy (in the MHz range).

“We’re coming up with a unique framework that incorporates different research perspectives to address how you use sound and vibration to treat and diagnose brain diseases,” explained Costas Arvanitis, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering and the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University. “Each researcher is bringing their own expertise to explore how vibrations and waves across a range of frequencies could either extract information from the brain or focus energy on the brain.”

Accessing the Brain Is a Tough Challenge

While it is possible to treat some tumors and other brain diseases non-invasively if they are near the center of the brain, many other conditions are harder to access, the researchers say.

“The center part of the brain is most accessible; however, even if you are able to target the part of the brain away from the center, you still have to go through the skull,” Arvanitis said.

He added that moving just 1 millimeter in the brain constitutes “a huge distance” from a diagnostic perspective. The science community widely acknowledges the brain’s complexity, each part associated with a different function and brain cells differing from one to the other.

According to Brooks Lindsey, a biomedical engineering assistant professor at Georgia Tech and Emory, there is a reason why brain imaging or therapy works well in some people but not in others.

“It depends on the individual patient’s skull characteristics,” he said, noting that some people have slightly more trabecular bone ? the spongy, porous part of the bone ? that makes it more difficult to treat.

Using ultrasound waves, the researchers are tackling the challenge on multiple levels. Lindsey’s lab uses ultrasound imaging to assess skull properties for effective imaging and therapy. He said his team conducted the first investigation that uses ultrasound imaging to measure the effects of bone microstructure — specifically, the degree of porosity in the inner, trabecular bone layer of the skull.

“By understanding transmission of acoustic waves through microstructure in an individual’s skull, non-invasive ultrasound imaging of the brain and delivery of therapy could be possible in a greater number of people,” he said, explaining one potential application would be to image blood flow in the brain following a stroke.

Refocusing Ultrasound Beams on the Fly

Arvanitis’ lab recently found a new way to focus ultrasound through the skull and into the brain, which is “100-fold faster than any other method,” Arvanitis said. His team’s work in adaptive focusing techniques would allow clinicians to adjust the ultrasound on the fly to focus it better.

“Current systems rely a lot on MRIs, which are big, bulky, and extremely expensive,” he said. “This method lets you adapt and refocus the beam. In the future this could allow us to design less costly, simpler systems, which would make the technology available to a wider population, as well as be able to treat different parts of the brain.”

Using ‘Guided Waves’ to Access Periphery Brain Areas

Another research cohort, led by Alper Erturk, Woodruff Professor of Mechanical Engineering at Georgia Tech, and former Georgia Tech colleague Massimo Ruzzene, Slade Professor of Mechanical Engineering at the University of Colorado Boulder, performs high-fidelity modeling of skull bone mechanics along with vibration-based elastic parameter identification. They also leverage guided ultrasonic waves in the skull to expand the treatment envelope in the brain. Erturk and Ruzzene are mechanical engineers by background, which makes their exploration of vibrations and guided waves in difficult-to-reach brain areas especially fascinating.

Erturk noted that guided waves are used in other applications such as aerospace and civil structures for damage detection. “Accurate modeling of the complex bone geometry and microstructure, combined with rigorous experiments for parameter identification, is crucial for a fundamental understanding to expand the accessible region of the brain,” he said.

Ruzzene compared the brain and skull to the Earth’s core and crust, with the cranial guided waves acting as an earthquake. Just as geophysicists use earthquake data on the Earth’s surface to understand the Earth’s core, so are Erturk and Ruzzene using the guided waves to generate tiny, high frequency “earthquakes” on the external surface of the skull to characterize what comprises the cranial bone.

Trying to access the brain periphery via conventional ultrasound poses added risks from the skull heating up. Fortunately, advances such as cranial leaky Lamb waves increasingly are recognized for transmitting wave energy to that region of the brain.

These cranial guided waves could complement focused ultrasound applications to monitor changes in the cranial bone marrow from health disorders, or to efficiently transmit acoustic signals through the skull barrier, which could help access metastases and treat neurological conditions in currently inaccessible regions of the brain.

Ultimately, the four researchers hope their work will make full brain imaging feasible while stimulating new medical imaging and therapy techniques. In addition to transforming diagnosis and treatment of brain diseases, the techniques could better detect traumas and skull-related defects, map the brain function, and enable neurostimulation. Researchers also see the potential for uncovering ultrasound-based blood-brain barrier openings for drug delivery for managing and treating diseases such as Alzheimer’s.

###

With this comprehensive research of the skull-brain system, and by understanding the fundamentals of transcranial ultrasound, the team hopes to make it more available to even more diseases and target many parts of the brain.

This work is funded by the National Science Foundation (CMMI Award 1933158 “Coupling Skull-Brain Vibroacoustics and Ultrasound Toward Enhanced Imaging, Diagnosis, and Therapy”).

CITATIONS: C. Sugino, M. Ruzzene, and A. Erturk, “Experimental and Computational Investigation of Guided Waves in a Human Skull.” (Ultrasound in Medicine and Biology, 2021) https://doi.org/10.1016/j.ultrasmedbio.2020.11.019

M. Mazzotti, E. Kohtanen, A. Erturk, and M. Ruzzene, “Radiation Characteristics of Cranial Leaky Lamb Waves.” (IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2021) https://doi.org/10.1109/TUFFC.2021.3057309

S. Schoen, C. Arvanitis, “Heterogeneous Angular Spectrum Method for Trans-Skull Imaging and Focusing.” (IEEE Xplore, 2020) https://ieeexplore.ieee.org/document/8902167

B. Jing, C. Arvanitis, B. Lindsey, “Effect of Incidence Angle and Wave Mode Conversion on Transcranial Ultrafast Doppler Imaging.” (IEEE Xplore, 2020) https://ieeexplore.ieee.org/document/9251477

The Georgia Institute of Technology, or Georgia Tech, is a top 10 public research university developing leaders who advance technology and improve the human condition.

The Institute offers business, computing, design, engineering, liberal arts, and sciences degrees. Its nearly 40,000 students, representing 50 states and 149 countries, study at the main campus in Atlanta, at campuses in France and China, and through distance and online learning.

As a leading technological university, Georgia Tech is an engine of economic development for Georgia, the Southeast, and the nation, conducting more than $1 billion in research annually for government, industry, and society.

https://rh.gatech.edu/news/646931/science-sound-vibration-better-diagnose-treat-brain-diseases

Source: https://bioengineer.org/the-science-of-sound-vibration-to-better-diagnose-treat-brain-diseases/

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Nontoxic, flexible energy converters could power wearable devices

Nontoxic, nanotube-based thermoelectric generation converts uneven heat distribution from wearables to electrical energy for their next cycle of operation.Credit: Injung

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Nontoxic, nanotube-based thermoelectric generation converts uneven heat distribution from wearables to electrical energy for their next cycle of operation.

WASHINGTON, April 27, 2021 — A wide variety of portable and wearable electronics have become a large part of our daily lives, so a group of Stanford University researchers wondered if these could be powered by harvesting electricity from the waste heat that exists all around us.

Further inspiration came from a desire to ultimately fabricate energy converting devices from the same materials as the active devices themselves, so they can blend in as an integral part of the total system. Today, many biomedical nanodevices’ power supplies come from several types of batteries that must be separated from the active portion of the systems, which is not ideal.

In Applied Physics Letters, from AIP Publishing, the researchers report the design and fabrication of single-wall carbon nanotube thermoelectric devices on flexible polyimide substrates as a basis for wearable energy converters.

“Carbon nanotubes are one-dimensional materials, known for good thermoelectric properties, which mean developing a voltage across them in a temperature gradient,” said Eric Pop, a professor of electrical engineering and materials science. “The challenge is that carbon nanotubes also have high thermal conductivity, meaning it’s difficult to maintain a thermal gradient across them, and they have been hard to assemble them into thermoelectric generators at low cost.”

The group uses printed carbon nanotube networks to tackle both challenges.

“For example, carbon nanotube spaghetti networks have much lower thermal conductivity than carbon nanotubes taken alone, due to the presence of junctions in the networks, which block heat flow,” Pop said. “Also, direct printing such carbon nanotube networks can significantly reduce their cost when they are scaled up.”

Thermoelectric devices generate electric power locally “by reusing waste heat from personal devices, appliances, vehicles, commercial and industrial processes, computer servers, time-varying solar illumination, and even the human body,” said Hye Ryoung Lee, lead author and a research scientist.

“To eliminate hindrances to large-scale application of thermoelectric materials — toxicity, materials scarcity, mechanical brittleness — carbon nanotubes offer an excellent alternative to other commonly used materials,” Lee said.

The group’s approach demonstrates a path to using carbon nanotubes with printable electrodes on flexible polymer substrates in a process anticipated to be economical for large-volume manufacturing. It is also “greener” than other processes, because water is used as the solvent and additional dopants are avoided.

Flexible and wearable energy harvesters can be embedded into fabrics or clothes or placed on unusual shapes and form factors.

“In contrast, traditional thermoelectrics that rely on bismuth telluride are brittle and stiff, with limited applications,” Pop said. “Carbon-based thermoelectrics are also more environmentally friendly than those based on rare or toxic materials like bismuth and tellurium.”

The most important concept in the group’s work is to “recycle energy as much as we can, converting uneven heat distribution to electrical energy for use for the next cycle of operation, which we demonstrated by using nontoxic nanotube-based thermoelectric generation,” said Yoshio Nishi, a professor of electrical engineering. “This concept is in full alliance with the world’s goal of reducing our total energy consumption.”

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The article “Carbon nanotube thermoelectric devices by direct printing: Towards wearable energy converters” is authored by Hye Ryoung Lee, Naoki Furukawa, Antonio J. Ricco, Eric Pop, Yi Cui, and Yoshio Nishi. The article will appear in Applied Physics Letters on April 27 (DOI: 10.1063/5.0042349). After that date, it can be accessed at https://aip.scitation.org/doi/10.1063/5.0042349.

ABOUT THE JOURNAL

Applied Physics Letters features rapid reports on significant discoveries in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See https://aip.scitation.org/journal/apl.

In Applied Physics Letters, from AIP Publishing, the researchers report the design and fabrication of single-wall carbon nanotube thermoelectric devices on flexible polyimide substrates as a basis for wearable energy converters.

Source: https://bioengineer.org/nontoxic-flexible-energy-converters-could-power-wearable-devices/

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Exposure to high heat neutralizes SARS-CoV-2 in less than one second

Texas A&M research shows exposure to high temperatures can neutralize the virus, preventing it from infecting another human hostCredit: Texas

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Texas A&M research shows exposure to high temperatures can neutralize the virus, preventing it from infecting another human host

Arum Han, professor in the Department of Electrical and Computer Engineering at Texas A&M University, and his collaborators have designed an experimental system that shows exposure of SARS-CoV-2 to a very high temperature, even if applied for less than a second, can be sufficient to neutralize the virus so that it can no longer infect another human host.

Applying heat to neutralize COVID-19 has been demonstrated before, but in previous studies temperatures were applied from anywhere from one to 20 minutes. This length of time is not a practical solution, as applying heat for a long period of time is both difficult and costly. Han and his team have now demonstrated that heat treatment for less than a second completely inactivates the coronavirus — providing a possible solution to mitigating the ongoing spread of COVID-19, particularly through long-range airborne transmission.

The Medistar Corporation approached leadership and researchers from the College of Engineering in the spring of 2020 to collaborate and explore the possibility of applying heat for a short amount of time to kill COVID-19. Soon after, Han and his team got to work, and built a system to investigate the feasibility of such a procedure.

Their process works by heating one section of a stainless-steel tube, through which the coronavirus-containing solution is run, to a high temperature and then cooling the section immediately afterward. This experimental setup allows the coronavirus running through the tube to be heated only for a very short period of time. Through this rapid thermal process, the team found the virus to be completely neutralized in a significantly shorter time than previously thought possible. Their initial results were released within two months of proof-of-concept experiments.

Han said if the solution is heated to nearly 72 degrees Celsius for about half a second, it can reduce the virus titer, or quantity of the virus in the solution, by 100,000 times which is sufficient to neutralize the virus and prevent transmission.

“The potential impact is huge,” Han said. “I was curious of how high of temperatures we can apply in how short of a time frame and to see whether we can indeed heat-inactivate the coronavirus with only a very short time. And, whether such a temperature-based coronavirus neutralization strategy would work or not from a practical standpoint. The biggest driver was, ‘Can we do something that can mitigate the situation with the coronavirus?’”

Their research was featured on the cover of the May issue of the journal Biotechnology and Bioengineering.

Not only is this sub-second heat treatment a more efficient and practical solution to stopping the spread of COVID-19 through the air, but it also allows for the implementation of this method in existing systems, such as heating, ventilation and air conditioning systems.

It also can lead to potential applications with other viruses, such as the influenza virus, that are also spread through the air. Han and his collaborators expect that this heat-inactivation method can be broadly applied and have a true global impact.

“Influenza is less dangerous but still proves deadly each year, so if this can lead to the development of an air purification system, that would be a huge deal, not just with the coronavirus, but for other airborne viruses in general,” Han said.

In their future work, the investigators will build a microfluidic-scale testing chip that will allow them to heat-treat viruses for much shorter periods of time, for example, tens of milliseconds, with the hope of identifying a temperature that will allow the virus to be inactivated even with such a short exposure time.

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The lead authors of the work are electrical engineering postdoctoral researchers, Yuqian Jiang and Han Zhang. Other collaborators on this project are Professor Julian L. Leibowitz, and Associate Professor Paul de Figueiredo from the College of Medicine; biomedical postdoctoral researcher Jose A. Wippold; Jyotsana Gupta, associate research scientist in microbial pathogenesis and immunology; and Jing Dai, electrical engineering assistant research scientist.

This work has been supported by grants from Medistar Corporation. Several research personnel on the project team were also supported by grants from the National Institutes of Health’s National Institute of Allergy and Infectious Diseases.

YouTube video link: https://youtu.be/noke1baewDs

YouTube video caption: Sub-second heat treatment of coronavirus

Video credit: Texas A&M University College of Engineering

Journal link: https://onlinelibrary.wiley.com/toc/10970290/2021/118/5

https://today.tamu.edu/2021/04/26/exposure-to-high-heat-neutralizes-sars-cov-2-in-less-than-one-second/

Their process works by heating one section of a stainless-steel tube, through which the coronavirus-containing solution is run, to a high temperature and then cooling the section immediately afterward. This experimental setup allows the coronavirus running through the tube to be heated only for a very short period of time. Through this rapid thermal process, the team found the virus to be completely neutralized in a significantly shorter time than previously thought possible. Their initial results were released within two months of proof-of-concept experiments.

Source: https://bioengineer.org/exposure-to-high-heat-neutralizes-sars-cov-2-in-less-than-one-second/

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